Internal combustion engines emit a large amount of unburned hydrocarbons during cold engine start-up. In fact, a substantial fraction of the emitted hydrocarbons have been found to occur during the first ten seconds due to the rich fuel mixture.
Release of hydrocarbons immediately after starting an engine poses a special problem because at this point the temperature of the conventional catalytic reactor is not high enough for conversion to innocuous products. The catalysts utilized in catalytic converter systems are generally ineffective at ambient temperature and must reach high temperatures, often several hundred degrees e.g., 300-400.degree. C. before they are activated.
One method of reducing cold start emissions is to temporarily adsorb hydrocarbons on zeolites which desorb at engine operating temperature and are converted by the main body catalyst.
One disadvantage of this method is that specific zeolites do not adsorb with equal efficiency at the full temperature range of the exhaust gas at start-up, that is, from room temperature to about 300.degree. C. Additionally, the adsorbed hydrocarbons can prematurely desorb before the main conversion catalyst is up to operating temperature, thus emitting hydrocarbons to the atmosphere.
It would be desirable to have a way to efficiently adsorb hydrocarbons without premature desorption at engine start-up. It would also be desirable to maintain the adsorption activity over the age of the adsorber.
The present invention fills these needs.